Tetramic acid chemistry. Part 1. Reinvestigation of racemisation during the synthesis of tetramic acids via Dieckmann cyclisation

Epimerisation during the synthesis of tetramic acids by Dieckmann cyclisation of the corresponding chiral N-acyl-α-amino esters was investigated. In the case of the isoleucine derivative, the extent of epimerisation was directly evaluated by 1H NMR analysis of the 3-acylated tetramic acids (3a, b) and (5a). A chemical correlation was carried out in the case of the 5-benzyl derivative (8h). The moderate overall yield and the partial epimerisation at position C-5 limit the usefulness of this approach for tetramic acid preparation.